1. Field of the Invention
The present invention relates to heat generating pipes that use the skin effect of an alternating current to allow heating of the entire pipe and yet have electrical conduction in only a small area.
2. Description of the Prior Art
It has long been known that it is highly desirable to heat a pipeline through which viscous fluids such as crude oil are being pumped. This greatly reduced the power and complexity of the pumps required to pump the fluids. There are several techniques that have been used to do this. A common example was steam tracing where a steam line was attached to the process pipe that was to be heated.
One other technique used to heat the pipeline was a skin effect heat generating pipe. This general concept was disclosed in U.S. Pat. No. 3,293,407. This patent disclosed the use of a ferromagnetic pipe with an internal non-magnetic electrical return conductor. This technique used alternating current which flowed only on the inside surface of the ferromagnetic pipe due to the skin effect. The pipe was heated as a result of electrical resistance, hysteresis and eddy current effects. This heat was thermally conducted to the outside of the pipe which remained at ground potential because of the skin effect. This conducted heat was transferred to the liquid if the pipe was inside the pipeline or was conducted to the pipe that comprised the pipeline and then transferred to the fluid. There were several disadvantages to this simple and straightforward technique. As pipelines grew longer the energizing voltage required to maintain a given power output per unit length increased. At certain pipeline lengths the required voltage exceeded the dielectric strength of the preferred insulations used on the internal return conductor.
One technique that was used to partially solve this problem was disclosed in U.S. Pat. No. 3,718,804. This patent disclosed the provision of a small gap between the heat generating pipe and the process pipe. The purpose of this gap was for arc suppression in case the dielectric of the return conductor broke down and began arcing over to the ferromagnetic heat generating pipe. The gap was provided to prevent arcing and the concomitant boring of holes in the process pipe which could lead to catastrophic results. This was, however, only a partial solution that alleviated one of the more severe possible results of insufficient insulation.
Another attempt to solve the insulation problem was disclosed in U.S. Pat. No. 3,524,996. This reference disclosed the use of a step-wise reduction in the dielectric strength of the insulation as the distances from the alternating current power supply source increased. This could be done because of the voltage difference generated in the ferromagnetic pipe and the electric conductor due to resistance, therefore reducing the voltage difference between the ferromagnetic pipe and the return electric conductor and reducing the needed dielectric strength of the insulation on the return conductor.
Other efforts in this area have concerned locally controlling the heat generation of a section of heat generating pipe. One example is evidenced in U.S. Pat. No. 3,575,581. This reference disclosed the use of a low resistance internal conductor to short circuit the internal conductive surface of the ferromagnetic pipe. A similar technique with an external shorting conducting was disclosed in U.S. Pat. No. 4,142,093.
Another alternative was disclosed in U.S. Pat. No. 4,110,599. In this reference a section of the ferromagnetic pipe was removed and replaced with an electrically conductive and non-ferromagnetic pipe having a significantly lesser heating effect. Yet another approach was disclosed in U.S. Pat. No. 4,132,884. This reference disclosed removing a section of the ferromagnetic pipe and replacing it with a non-conductive section. An additional insulated internal conductor would be used to conduct the current across the nonconducting section and between the two adjacent sections of ferromagnetic pipe.
These references were only intended to stop the heating in a given area and were not intended to reduce or solve any voltage or dielectric strength problems.
U.S. Pat. No. 4,256,945 disclosed the use of a substrate made of a non-magnetic material having high thermal and electrical conductivity clad with a surface layer of ferromagnetic material of relatively low electrical conductivity. This composite was used in a heating element. A high frequency power source was applied to the heating element to cause the ferromagnetic material to conduct and heat up. As the temperature of the ferromagnetic material increased, the temperature would rise into the region of the Curie temperature of the ferromagnetic material where the magnetic permeability of the ferromagnetic material would decline. This decline in the permeability would lessen the skin effect, allowing the current to migrate into the higher conductivity and non-magnetic core, thereby causing a reduction in the heating effect and a cooling of the ferromagnetic material. This cooling would bring the ferromagnetic material temperature below the Curie temperature and the permeability would increase allowing the skin effect to increase, concentrating the current in the ferromagnetic material. Therefore it can be seen that this reference disclosed the use of the two materials to produce a constant temperature heating element based on the Curie temperature of the coating ferromagnetic material. This method was not suitable for a skin effect heat generating pipe in a standard application because the conductive layer was on the outside whereas in standard heat generating pipe the conductive layer of the ferromagnetic material is on the inside of the pipe and the outside surface is at ground potential. Further, the standard heat generating pipe requires a ferromagnetic material thickness several times greater than the skin effect depth, which was not true of the system disclosed in this patent. A final difference is that heat generating pipes preferably operate at low frequencies, generally 50 or 60 Hz, not at the higher frequencies disclosed in this reference.